The present invention relates to a high resolution contact lens structure, for use with a long working distance microscope objective, for examining a cornea of an eye. Contact lens structures are sometimes referred to as dipping cones.
Within this application several publications are references by arabic numerals within parentheses. Full citations for these and other references may be found at the end of the specification immediately preceding the claims. The disclosures of all of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
High magnification, clinical microscopy of the cornea has been made practical by the use of optical elements that contact the surface (1), thereby establishing focus and damping involuntary lateral movements of the eye. Specular microscopes, used for examination of the endothelial cell layer of the cornea, make use of a so-called dipping cone made of a single piece of glass that attaches to the front of a long working distance microscope objective.
Generally, a dipping cone employs a flat front surface and a spherical rear surface that is concentric to the focal plane. This design increases the numerical aperture ("NA") of the system by a factor equal to the refractive index of the cone material, typically 1.52 to 1.65. The aberrations introduced are negligible for low NA systems, typically NA 0.35. However, dipping cones of this design cannot be used at higher NA because of aberrations introduced at the plane front surface. While an NA of 0.35 is sufficient for endothelial and superficial epithelial cell examinations, a higher resolution is desirable for studies of detail within the stroma (the central layer of the cornea), cell boundaries within the epithelial cell layer, nerves in the cornea, etc.
For examination of subtle corneal detail it is also necessary to utilize optical sectioning, in order to reject scattered and reflected light from levels other than the plane of interest. This has been accomplished by scanning slits(2,3) and more recently by the tandem scanning microscope that utilizes arrays of pinhole apertures(4). Sources of reflection and scattered light that can contribute stray light to the image include reflection from the interface between the cornea and the contact element, and even reflections from the endothelial cell layer when the posterior stroma is being examined. Furthermore, out-of-focus cells (keratocytes) in the cornea can produce an undesirable blurred background that can obscure the details of in-focus keratocytes.